A servo system is a control system that controls the operation of an apparatus through a series of feedback signals to the control system. The feedback signal is typically from a sensor that determines the offset of the actual operation of the apparatus from the requested operation of the apparatus. Some examples of servo systems are: a thermostat for controlling the temperature of a room; an autopilot for controlling the flying attitude and direction of an aircraft; a cruise control for controlling the speed of an automobile; a servo system in a hard disk drive (HDD) for controlling disk RPM and magnetic transducer position.
In a more sophisticated and precise apparatus such as an HDD, a servo system will also use a feed-forward signal. A feed-forward signal in an HDD provides control signals for actuating a voice coil motor, based upon a reference model stored in a control module. The reference model is based upon a nominal design of the HDD. In actual operation of the HDD the feed-forward signal from the reference model can be inaccurate in predicting control signals for actuating the voice coil motor due to tolerances in the numerous components that determine the performance of the voice coil motor. Inaccuracy in the feed-forward signal will cause a feedback signal from the magnetic transducer to compensate for the feed-forward inaccuracy and attempt to bring the magnetic transducer onto the desired data track.
The amount of data that can be stored on a disk is governed by many well-known physical principles. One factor in determining the amount of data that can be stored in an HDD is the ability of the magnetic transducer to write closely spaced data tracks onto the disk surface. The spacing of tracks on a disk surface is known as track pitch, and the unit of measure for expressing the density of tracks on a disk is tracks per inch or TPI. A smaller track pitch results in a higher TPI. The servo system of the HDD enables in part the magnetic transducer to be precisely positioned at a radius on the disk to write a data track, enabling adjacent data tracks to be written with a high TPI.
Once the data track is written, the HDD must be able to find the track and settle upon it in a minimal amount of time. This amount of time is typically known as seek-settle time, and when coupled with the time for the data to rotate to where it can be read and the time required for the HDD to decipher the data on the track, this total time is known as latency. An HDD user usually prefers short latency. The robustness of the servo system and accuracy of the feed-forward signal will determine if the magnetic transducer has settled on a desired data track or needs to make another attempt at settling on the data track.
An HDD is one example of a mechanism using a servo system. One of ordinary skill in the art will appreciate that embodiments presented are beneficial to a variety of mechanisms operable to a control or servo system. The HDD will be used for the sake of brevity and clarity to demonstrate the need for a well-controlled servo system and a servo system's vulnerability to problems.